Codebase hardening: 102 fixes across 35+ files

Deep audit identified 106 findings; 102 fixed, 4 deferred. Covers 8 areas:

- Settings & deploy: env-gated DEBUG/SECRET_KEY, HTTPS headers, gunicorn, celery worker
- Auth (registered_user): password write_only, request.data fixes, transaction safety, proper HTTP status codes
- Workout app: IDOR protection, get_object_or_404, prefetch_related N+1 fixes, transaction.atomic
- Video/scripts: path traversal sanitization, HLS trigger guard, auth on cache wipe
- Models (exercise/equipment/muscle/superset): null-safe __str__, stable IDs, prefetch support
- Generator views: helper for registered_user lookup, logger.exception, bulk_update, transaction wrapping
- Generator core (rules/selector/generator): push-pull ratio, type affinity normalization, modality checks, side-pair exact match, word-boundary regex, equipment cache clearing
- Generator services (plan_builder/analyzer/normalizer): transaction.atomic, muscle cache, bulk_update, glutes classification fix

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

superset/serializers.py

@@ -2,33 +2,33 @@ from rest_framework import serializers
 from .models import *
 from exercise.models import Exercise
 from exercise.serializers import ExerciseSerializer
-import uuid
 
 class SupersetExerciseSerializer(serializers.ModelSerializer):
     exercise = serializers.SerializerMethodField()
     unique_id = serializers.SerializerMethodField()
-    
+
     class Meta:
         model = SupersetExercise
         fields = '__all__'
-    
-    def get_exercise(self, obj): 
+
+    def get_exercise(self, obj):
         data = ExerciseSerializer(obj.exercise, many=False).data
         return data
-    
-    def get_unique_id(self, obj): 
-        return str(uuid.uuid4())
+
+    def get_unique_id(self, obj):
+        return f"{obj.pk}-{obj.superset_id}" if hasattr(obj, 'superset_id') else str(obj.pk)
 
 class SupersetSerializer(serializers.ModelSerializer):
     exercises = serializers.SerializerMethodField()
-    
+
     class Meta:
         model = Superset
         fields = '__all__'
-    
+
     def get_exercises(self, obj):
         if obj.pk is None:
             return []
-        objs = SupersetExercise.objects.filter(superset=obj).order_by('order')
+        # Use prefetched data if available via superset_exercises related manager
+        objs = obj.superset_exercises.all().order_by('order')
         data = SupersetExerciseSerializer(objs, many=True).data
         return data